Personally, I think leaving it as "So this is a woman's..." is fine because of the ambiguity. "These are a woman's..." only implies breasts, whereas "This is a woman's..." can be taken to mean "This is a woman's chest", "This is a woman's pair of tits", or "This is a woman's (naked) body..." Considering that the next panels are her realizing that it's her body "...But this is my own body!" and her enjoying the full view of her body and not just her breasts, the full body interpretation could be better than focusing entirely on the boobs.
As for the original grammar, Japanese is fairly vague when it comes to plurality so the original これが女の can be translated to both "This is..." and "These are..." depending on the context. There is a more explicitly plural これら for "these" but I don't think I've really seen the plural これら used with おっぱい. But without the full context of the subject, これが usually gets translated as the singular "This is..."
Honestly though, I think either one would be good enough. It just really depends on what the reader thinks comes next in that unfinished sentence.